Method and system for encoding multi-level pulse amplitude modulated signals using integrated optoelectronic devices

ABSTRACT

Methods and systems for encoding multi-level pulse amplitude modulated signals using integrated optoelectronics are disclosed and may include generating a multi-level, amplitude-modulated optical signal utilizing an optical modulator driven by first and second electrical input signals, where the optical modulator may configure levels in the multi-level amplitude modulated optical signal, drivers are coupled to the optical modulator; and the first and second electrical input signals may be synchronized before being communicated to the drivers. The optical modulator may include optical modulator elements coupled in series and configured into groups. The number of optical modular elements and groups may configure the number of levels in the multi-level amplitude modulated optical signal. Unit drivers may be coupled to each of the groups. The electrical input signals may be synchronized before communicating them to the unit drivers utilizing flip-flops. Phase addition may be synchronized utilizing one or more electrical delay lines.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of application Ser. No. 16/424,137filed on May 28, 2019, which is a continuation of application Ser. No.15/407,440 filed on Jan. 17, 2017, now U.S. Pat. No. 10,305,597, whichis a continuation of application Ser. No. 14/196,122 filed on Mar. 4,2014, now U.S. Pat. No. 9,548,811, which is a continuation ofapplication Ser. No. 13/568,616 filed on Aug. 7, 2012, now U.S. Pat. No.8,665,508, which is a continuation of application Ser. No. 12/555,291filed on Sep. 8, 2009, now U.S. Pat. No. 8,238,014, which in turn makesreference to, claims priority to and claims the benefit of: U.S.Provisional Patent Application No. 61/191,480 filed on Sep. 8, 2008.

This application also makes reference to U.S. Pat. No. 7,039,258.

Each of the above stated applications is hereby incorporated herein byreference in its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

[MICROFICHE/COPYRIGHT REFERENCE]

[Not Applicable]

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing. Morespecifically, certain embodiments of the invention relate to a methodand system for encoding multi-level pulse amplitude modulated signalsusing integrated optoelectronics.

BACKGROUND OF THE INVENTION

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for encoding multi-level pulse amplitudemodulated signals using integrated optoelectronics, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram of an exemplary split domain Mach-Zehndermodulator, in accordance with an embodiment of the invention.

FIG. 3 is a schematic of an exemplary multi-level pulse-amplitudemodulated Mach-Zehnder interferometer, in accordance with an embodimentof the invention.

FIG. 4 is a schematic of an exemplary multi-level pulse-amplitudemodulated Mach-Zehnder interferometer with associated electronics forhigh-speed optical modulation, in accordance with an embodiment of theinvention

FIG. 5 is a flow chart illustrating exemplary steps in the operation ofa multi-level pulse-amplitude modulated Mach-Zehnder interferometer, inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a method and system forencoding multi-level pulse amplitude modulated signals using integratedoptoelectronics. Exemplary aspects of the invention may comprisegenerating a multi-level, amplitude-modulated optical signal utilizingan optical modulator driven by two or more of a plurality of electricalinput signals. The optical modulator may comprise a plurality of opticalmodulator elements coupled in series and configured into a plurality ofgroups. The number of the optical modular elements and the plurality ofgroups may configure the number of levels in the multi-level amplitudemodulated optical signal. Unit drivers may be coupled to each of theplurality of groups of the optical modulator elements. The plurality ofelectrical input signals may be synchronized before communicating thesignals to the unit drivers utilizing flip-flops. Two or more of theplurality of electrical input signals may be selected utilizing one ormore multiplexers, which may select an electrical input or a complementof the electrical input. Phase addition may be synchronized in theplurality of optical modulator elements utilizing one or more electricaldelay lines. The optical modulator may be integrated on a singlesubstrate, which may comprise one of: silicon, gallium arsenide,germanium, indium gallium arsenide, or indium phosphide. The opticalmodulator may comprise a Mach-Zehnder interferometer and/or one or morering modulators.

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention. Referring to FIG. 1A,there is shown optoelectronic devices on a CMOS chip 130 comprising highspeed optical modulators 105A-105D, high-speed photodiodes 111A-111D,monitor photodiodes 113A-113H, and optical devices comprising taps103A-103K, optical terminations 115A-115D, and grating couplers117A-117H. There is also shown electrical devices and circuitscomprising transimpedance and limiting amplifiers (TIA/LAs) 107A-107E,analog and digital control circuits 109, and control sections 112A-112D.Optical signals are communicated between optical and optoelectronicdevices via optical waveguides fabricated in the CMOS chip 130.

The high speed optical modulators 105A-105D comprise Mach-Zehnder orring modulators, for example, and enable the modulation of the CW laserinput signal. The high speed optical modulators 105A-105D are controlledby the control sections 112A-112D, and the outputs of the modulators areoptically coupled via waveguides to the grating couplers 117E-117H. Thetaps 103D-103K comprise four-port optical couplers, for example, and areutilized to sample the optical signals generated by the high speedoptical modulators 105A-105D, with the sampled signals being measured bythe monitor photodiodes 113A-113H. The unused branches of the taps103D-103K are terminated by optical terminations 115A-115D to avoid backreflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D are utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H areutilized to couple light from the CMOS chip 130 into optical fibers. Theoptical fibers may be epoxied, for example, to the CMOS chip, and may bealigned at an angle from normal to the surface of the CMOS chip 130 tooptimize coupling efficiency.

The high-speed photodiodes 111A-111D convert optical signals receivedfrom the grating couplers 117A-117D into electrical signals that arecommunicated to the TIA/LAs 107A-107D for processing. The analog anddigital control circuits 109 may control gain levels or other parametersin the operation of the TIA/LAs 107A-107D. The TIA/LAs 107A-107D thencommunicate electrical signals off the CMOS chip 130.

The control sections 112A-112D comprise electronic circuitry that enablemodulation of the CW laser signal received from the splitters 103A-103C.The high speed optical modulators 105A-105D require high-speedelectrical signals to modulate the refractive index in respectivebranches of a Mach-Zehnder interferometer (MZI), for example. Thevoltage swing required for driving the MZI is a significant power drainin the CMOS chip 130. Thus, if the electrical signal for driving themodulator may be split into domains with each domain traversing a lowervoltage swing, power efficiency is increased.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention. Referring to FIG. 1B, there isshown the CMOS chip 130 comprising electronic devices/circuits 131,optical and optoelectronic devices 133, a light source interface 135,CMOS chip surface 137, an optical fiber interface 139, and CMOS guardring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers that enable coupling of light signals via theCMOS chip surface 137, as opposed to the edges of the chip as withconventional edge-emitting devices. Coupling light signals via the CMOSchip surface 137 enables the use of the CMOS guard ring 141 whichprotects the chip mechanically and prevents the entry of contaminantsvia the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theTIA/LAs 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the taps 103A-103K,optical terminations 115A-115D, grating couplers 117A-117H, high speedoptical modulators 105A-105D, high-speed photodiodes 111A-111D, andmonitor photodiodes 113A-113H.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.Referring to FIG. 1C, there is shown the CMOS chip 130 comprising theelectronic devices/circuits 131, the optical and optoelectronic devices133, the light source interface 135, the CMOS chip surface 137, and theCMOS guard ring 141. There is also shown a fiber to chip coupler 143, anoptical fiber cable 145, and a light source module 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an embodiment of the invention, the optical fiber cable may beaffixed, via epoxy for example, to the CMOS chip surface 137. The fiberchip coupler 143 enables the physical coupling of the optical fibercable 145 to the CMOS chip 130.

The light source module 147 may be affixed, via epoxy or solder, forexample, to the CMOS chip surface 137. In this manner a high power lightsource may be integrated with optoelectronic and electronicfunctionalities of one or more high-speed optoelectronic transceivers ona single CMOS chip.

A distributed Mach-Zehnder interferometer (MZI) comprises a number ofunit drivers each receiving an electrical signal and amplifying it todrive a separate optical modulating element in one of the MZI arms. Themodulating elements may use the electrical signal from the unit driversto create a phase shift in the optical carrier. Such a phase shift maybe directly additive as light travels from one modulating element to thenext, and may accumulate along each of the interferometer arms, allowingthe MZI to achieve a significant phase difference between the opticalsignals in the two arms. When the light is recombined, the resultingconstructive and destructive interference patterns may create atwo-level amplitude envelope which follows the applied electricalsignal. High-speed amplitude modulation may be achieved when theelectrical signals feeding the unit drivers are delayed relative to eachother to match the propagation delay of light in the MZI waveguides. Thehigh-amplitude optical output may represent logic ‘1’ and alow-amplitude output may represent logic ‘0’. Thus, one data bit may beconveyed by each unit interval of the waveform. By utilizing more thantwo amplitude levels, more bits per unit interval may be communicated.For example, PAM-4 sends two bits per unit interval, PAM-8 sends 3, andPAM-16 sends 4.

In an embodiment of the invention, the distributed MZI may be integratedon a single chip, such as the CMOS chip 130. The substrate may comprisesilicon, or other semiconductor material such as germanium, indiumphosphide, gallium arsenide, or indium gallium arsenide.

FIG. 2 is a block diagram of an exemplary split domain Mach-Zehndermodulator, in accordance with an embodiment of the invention. Referringto FIG. 2, there is shown a split-domain Mach-Zehnder modulator (MZM)250 comprising a transmission line driver 209, waveguides 211,transmission lines 213A-213D, diode drivers 215A-215H, diodes 219A-219D,and transmission line termination resistors RTL1-RTL4. There is alsoshown voltage levels Vdd, Vd, and Gnd. In an embodiment of theinvention, Vd is equal to a voltage of Vdd/2, thus generating twovoltage domains, due to the symmetric nature of the stacked circuits.However, the invention is not limited to two voltage domains.Accordingly, any number of voltage domains may be utilized, dependent onthe desired voltage swing of each domain and the total voltage range,defined here as Vdd to ground. Similarly, the magnitude of the voltagerange in each voltage domain may be a different value than otherdomains.

The transmission line (T-line) driver 209 comprises circuitry fordriving transmission lines in an even-coupled mode, where the signal oneach pair of transmission lines is equal except with a DC offset. Inthis manner, two or more voltage domains may be utilized to drive thediodes that generate index changes in the respective branches of the MZM250. In another embodiment of the invention, the T-line driver 209 maydrive transmission lines in odd-coupled mode. Even-coupled mode mayresult in a higher impedance in the transmission line, whereasodd-coupling may result in lower impedance.

The waveguides 211 comprise the optical components of the MZM 250 andenable the routing of optical signals around the CMOS chip 130. Thewaveguides 211 comprise silicon and silicon dioxide, formed by CMOSfabrication processes, utilizing the index of refraction differencebetween Si and SiO₂ to confine an optical mode in the waveguides 211.The transmission line termination resistors R_(TL1)-R_(TL4) enableimpedance matching to the T-lines 213A-213D and thus reducedreflections.

The diode drivers 215A-215H comprise circuitry for driving the diodes219A-219D, thereby changing the index of refraction locally in thewaveguides 211. This index change in turn changes the velocity of theoptical mode in the waveguides 211, such that when the waveguides mergeagain following the driver circuitry, the optical signals interfereconstructively or destructively, thus modulating the laser input signal.By driving the diodes 219A-219D with a differential signal, where asignal is driven at each terminal of a diode, as opposed to one terminalbeing tied to AC ground, both power efficiency and bandwidth may beincreased due to the reduced voltage swing required in each domain.

In operation, a CW optical signal is coupled into the “Laser Input”, anda modulating differential electrical signal is communicated to theT-line driver 209. The T-line driver 209 generates complementaryelectrical signals to be communicated over the T-lines 213A-213D, witheach pair of signals offset by a DC level to minimize the voltage swingof each diode driver 215A-215H, while still enabling a full voltageswing across the diodes 219A-219D.

Reverse biasing the diodes 219A-219D generates field effects that changethe index of refraction and thus the speed of the optical signalpropagating through the waveguides 213A-213D. The optical signals theninterfere constructively or destructively, resulting in the “ModulatedLight” signal.

A distributed Mach-Zehnder interferometer (MZI) comprises a number ofunit drivers each receiving an electrical signal and amplifying it todrive a separate optical modulating element in one of the MZI arms. Themodulating elements may use the electrical signal from the unit driversto create a phase shift in the optical carrier. Such a phase shift maybe directly additive as light travels from one modulating element to thenext, and may accumulate along each of the interferometer arms, allowingthe MZI to achieve a significant phase difference between the opticalsignals in the two arms. When the light is recombined, the resultingconstructive and destructive interference patterns may create atwo-level amplitude envelope which follows the applied electricalsignal. High-speed amplitude modulation may be achieved when theelectrical signals feeding the unit drivers are delayed relative to eachother to match the propagation delay of light in the MZI waveguides. Thehigh-amplitude optical output may represent logic ‘1’ and alow-amplitude output may represent logic ‘0’. Thus, one data bit may beconveyed by each unit interval of the waveform.

In an embodiment of the invention, the distributed MZI may be integratedon a single substrate and single chip. The substrate may comprisesilicon, or other semiconductor material such as germanium, indiumphosphide, gallium arsenide, or indium gallium arsenide.

FIG. 3 is a schematic of an exemplary multi-level pulse-amplitudemodulated Mach-Zehnder interferometer, in accordance with an embodimentof the invention. Referring to FIG. 3, there is shown multi-level PAMMZI 300 comprising optical phase modulators 301A and 301B, unit driversA 303, unit drivers B 305, a multiplexer (MUX) 307, and an opticalwaveguide 315. There is also shown input data streams Dat<1> 309,DatB<1> 311, Dat<0> 313, a CW laser input, and an optical output. Theoptical waveguide may be substantially similar to the waveguides 211described with respect to FIG. 2.

The optical phase modulators 301A and 301B may comprise sections of theoptical waveguide 315 and the unit drivers A 303 and B 305,respectively. The unit drivers A 303 and B 305 may comprise distributeddrivers, such as the diode drivers 215A-215H described with respect toFIG. 2, that may enable multi-level modulation directly in the opticaldomain. The modular nature of a distributed driver may enable thedivision into two or more banks of unit drivers, each receiving adifferent electrical signal, which can add or subtract optical carrierphase in each of the MZI arms via the optical modulators. Thus, as thenumber of unit drivers and their bandwidth tend to infinity, opticalwaveform envelopes of arbitrary shape and complexity may be generated.The MUX 307 may comprise a multiplexer for switching between desiredinputs Dat<1> and its binary complement DatB<1>. For higher order PAM,the multiplexing logic may require more inputs and outputs, andconsequently, more complexity.

In operation, the multi-level PAM MZI 300 may be enabled to modulate aCW laser input, generating a 4-level PAM optical output. The predominantapplication of optical modulators is in data communication. A modulatedwaveform may be subdivided into unit intervals, each representing one ormore bits, depending on the number of possible envelope levels.Two-level pulse amplitude modulation (PAM-2) is the most common, as itmaintains a large energy distance between two possible values (‘1’ or‘0’), which increases signal-to-noise ratio (SNR) and reduces theprobability of errors due to additive noise. High data throughput is oneof the key objectives in designing data communication systems, and oneoption for increased throughput is to reduce the duration of the unitinterval. However, this may be constrained by the circuit and modulatorbandwidth. In an embodiment of the invention, data throughput may beincreased by encoding multiple bits of information in each unitinterval. This may be accomplished by subdividing the available signalenergy into a higher number of discrete levels. This produces a PAM-Nmodulation, where N is the number of levels and In(N)/In(2) is thenumber of data bits in each unit interval. PAM encoding may beaccomplished in the electrical domain using digital to analog converters(DACs) followed by linear amplifiers. Due to DAC settling requirementsand linear amplifier gain and bandwidth variations across process,voltage, and temperature (PVT), such blocks are challenging to designfor high-speed operation.

In an embodiment of the invention, analog circuit complexity may bereduced by operating with 2-level binary signals in the electricaldomain while creating multi-level signals in the optical domain. Forsimplicity, FIGS. 3 and 4 illustrate PAM-4 embodiments, but theinvention need not be so limited. Higher-order PAM may be implementeddepending on the desired data throughput and SNR. The multi-level PAMMZI 300 may receive two parallel binary data streams Dat<0> and Dat<1>.Their complements DatB<0> and DatB<1> are also available, as is the casein high-speed differential logic circuits.

The unit driver for the multi-level PAM MZI 300 may be divided into twobanks. The first bank, unit drivers A 303, may comprise ⅔ of the totalnumber of unit drivers, for example, while the second bank, unit driversB 305, may comprise ⅓ of the unit drivers, each unit driver beingconnected to a dedicated pair of optical modulating element—one in eachMZI arm. The unit drivers A 303 and B 305 may feature differentialoutputs to drive each MZI arm with electrical signals which are 180° outof phase to maximize the phase difference in both arms. Dat<1> may besent to the unit drivers A 303 while Dat<0> may be used to control theMUX 307 which sends either Dat<1> or its binary complement DatB<1> tothe unit drivers B 305.

In the optical domain, Dat<1> may create a positive or negative opticalcarrier phase shift in each of the MZI arms. The phase shift of the unitdrivers B 305 may be added to or subtracted from the phase shiftgenerated by the unit drivers A 303. Assuming that all unit drivers andtheir respective optical phase modulators are identical, the amount ofphase shift generated by each bank may be proportional to the number ofelements it contains. Thus ⅔ and ⅓ banks can create 4 levelscorresponding to 0, ⅓, ⅔, and 1 of the total available envelope range,which correspond to binary numbers of 00, 01, 10, and 11 respectively.Thus, each unit interval in the optical domain contains two bits ofinformation. Accordingly, any number of unit drivers may be utilized toconfigure the total number of PAM output levels.

FIG. 4 is a schematic of an exemplary multi-level pulse-amplitudemodulated Mach-Zehnder interferometer with associated electronics forhigh-speed optical modulation, in accordance with an embodiment of theinvention. Referring to FIG. 4, there is shown a multi-level PAM MZI400, which may be substantially similar to the multi-level PAM MZI 300,but with the added functionality provided by the flip-flops 401, thedrivers 403A and 403B, and the electrical delay line 405.

The flip-flops 401 may comprise D-flip-flops, for example, which mayenable the synchronization of data signals Dat<1> and the output of theMUX 307 based on a received clock signal, CLK. The drivers 403 maycomprise amplifiers for providing gain to the synchronized signalsbefore being communicated to the unit drivers A 303 and B 305. Theelectrical delay line 405 may be operable to delay the electrical signalfrom the driver 403B and may enable better synchronization of modulationby the optical phase modulators 301A and 301 B. The electrical delayline 405 may be either passive, such as a transmission line, or active,comprising transistor circuits, for example.

In operation, the data streams derived from inputs Dat<1>/DatB<1> andDat<0>/DatB<0> may be synchronized by the flip-flops 401 andpost-amplified by the drivers 403A and 403B before being sent to theunit drivers A 303 and B 305. In addition, the electrical delay line 405may be utilized on the path to the more distant ⅓-sized modulator bank,the unit drivers B 305. The electrical delay line 405 may be designed tomach the delay of light propagating through the first ⅔-sized modulatorbank, the optical phase modulator 301A. This enables high-speedmodulation by providing a temporal alignment between the optical carrierphase transitions arriving at each modulating element and the phasetransitions contributed by that element, thereby enabling linear phaseaddition at high data rates.

In another embodiment of the invention, the multi-level PAM MZI 400 mayinstead comprise one or more ring modulators . In an embodiment of theinvention, ring modulators may be utilized to replace linear modulatorsections in a Mach-Zehnder modulator. Similarly, the ring modulators maycomprise a plurality of modulator elements being configured in groups,with the number of groups as well as the number of elements within thegroup determining the levels in the PAM-N modulation.

FIG. 5 is a flow chart illustrating exemplary steps in the operation ofa multi-level pulse-amplitude modulated Mach-Zehnder interferometer, inaccordance with an embodiment of the invention. In step 503, after startstep 501, electrical data signals are received, selected, synchronized,and amplified followed by end step 511. In step 505, one of theselected, synchronized, and amplified signals may be utilized to driveone or more optical modulator sections that modulates a CW laser inputoptical signal, while a second set of one or more selected,synchronized, and amplified signals may be delayed utilizing anelectrical delay line. In step 507, the modulated optical signal may befurther modulated utilizing the one or more delayed electrical signals,and the PAM-N modulated signal may be output in step 509, followed byend step 511

In an embodiment of the invention, a method and system are disclosed forencoding multi-level pulse amplitude modulated signals using integratedoptoelectronics. Aspects of the invention may comprise generating amulti-level, amplitude-modulated optical signal utilizing an opticalmodulator 300/400 driven by two or more of a plurality of electricalinput signals Dat<1>, DatB<1>, Dat<0>. The optical modulator 300/400 maycomprise a plurality of optical modulator elements coupled in series andconfigured into a plurality of groups 301A/301B. The number of theoptical modular elements and the plurality of groups may configure thenumber of levels in the multi-level amplitude modulated optical signal.Unit drivers 303/305 may be coupled to each of the plurality of groupsof the optical modulator elements. The plurality of electrical inputsignals Dat<1>, DatB<1>, Dat<0> may be synchronized before communicatingthe signals to the unit drivers utilizing flip-flops 401. Two or more ofthe plurality of electrical input signals may be selected utilizing oneor more multiplexers 307, which may select an electrical input Dat<1> ora complement of the electrical input DatB<1>. Phase addition may besynchronized in the plurality of optical modulator elements utilizingone or more electrical delay lines 405. The optical modulator 300/400may be integrated on a single substrate, which may comprise one of:silicon, gallium arsenide, germanium, indium gallium arsenide, indiumphosphide, or polymer-based materials. The optical modulator maycomprise a Mach-Zehnder interferometer 300/400 or one or more ringmodulators.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

1. (canceled)
 2. A system for processing signals, the system comprising:a plurality of flip-flops arranged to synchronize electrical inputsignals; a plurality of drivers arranged to drive the synchronizedelectrical input signals from the plurality of flip-flops; and anoptical modulator arranged to generate a multi-level,amplitude-modulated optical signal based on the synchronized electricalinput signals from the plurality of drivers.
 3. The system of claim 2,further comprising one or more multiplexers arranged to select theelectrical input signals.
 4. The system of claim 3, wherein the one ormore multiplexers are arranged to select between an electrical input ora complement of the electrical input.
 5. The system of claim 2, furthercomprising one or more delay lines used to synchronize phase addition ina plurality of optical modulator elements in the optical modulator. 6.The system of claim 2, wherein the optical modulator is integrated on asingle substrate.
 7. The system of claim 6, wherein the single substratecomprises one of: silicon, gallium arsenide, germanium, indium galliumarsenide, indium phosphide, or polymer-based materials.
 8. The system ofclaim 2, wherein the optical modulator comprises a Mach-Zehnderinterferometer.
 9. The system of claim 2, wherein the optical modulatorcomprises one or more ring modulators.
 10. A system for processingsignals, the system comprising: an optical modulator arranged to use afirst electrical input signal and a second electrical input signal todrive a plurality of diodes to change an index of refraction ofwaveguides in the optical modulator and to generate a multi-level,amplitude modulated optical signal; and a plurality of drivers arrangedto provide the first electrical input signal and the second electricalinput signal to the optical modulator.
 11. The system of claim 10,further comprising one or more multiplexers arranged to select the firstelectrical input signal and the second electrical input signal.
 12. Thesystem of claim 11, wherein the one or more multiplexers are arranged toselect between an electrical input or a complement of the electricalinput.
 13. The system of claim 10, further comprising one or more delaylines used to synchronize phase addition in a plurality of opticalmodulator elements in the optical modulator.
 14. The system of claim 10,wherein the optical modulator is integrated on a single substrate. 15.The system of claim 14, wherein the single substrate comprises one of:silicon, gallium arsenide, germanium, indium gallium arsenide, indiumphosphide, or polymer-based materials.
 16. The system of claim 10,wherein the optical modulator comprises a Mach-Zehnder interferometer.17. The system of claim 10, wherein the optical modulator comprises oneor more ring modulators.